The present invention relates to a circulating fluidized bed combustor or gasifier for application in pressurized combustion or gasification systems, the systems comprising at least one upright combustion chamber and one particle separator connected thereto enclosed in a common external upright pressure vessel.
In conventional circulating fluidized bed processes high flow velocity and excellent mixing of particles and gases leads to efficient heat transfer and improved combustion efficiency. SO2 and NOx emissions are low due to desulphurizing sorbents used and due to staged combustion. Various fuels and refuse derived wastes may be burned or gasified and utilized in circulating fluidized bed combustion. The temperature is very stable and the heat transfer rate is high.
In pressurized circulating fluidized bed processes principally all advantages from atmospheric circulating fluidized bed processes are maintained, whereas some additional advantages are achieved.
The size of a pressurized steam generation plant, including combustion chamber and particle separators, can be made much smaller than a corresponding conventional atmospheric steam generation plant. Significant savings in material and investment costs are achieved.
Further pressurized steam generation systems provide increased total efficiency compared to atmospheric steam boilers. Pressurizing of a circulating fluidized bed process provides a considerable increase in efficiency/volume ratio.
In pressurized circulating fluidized bed systems fuel is combusted or gasified in a combustion chamber at high temperatures and high pressure. The external vessel provides pressure containment, which is cooled or insulated to enhance material strength and to thereby minimize costs of the pressure vessel. Combustion air pressurized in a compressor is directed into the pressure vessel into the space between the combustor and the peripheral wall of the pressure vessel. The pressurized air thereby provides for cooling of the walls of the pressure vessel. In the vessel the pressurized air is further directed through a grid into the combustion chamber for fluidizing and combusting of material therein. The pressure in the pressure vessel may be 8-30 bar, typically 10-14 bar.
In a circulating fluidized bed system particles are separated in a particle separator, such as a cyclone or hot gas filter, from the hot gases produced in the combustion chamber and the separated particles are recycled into the combustion chamber. In a combined gas/steam power plant the hot gases discharged from the particle separator may be further cleaned and utilized in a gas turbine, thereby increasing the electrical efficiency of the power plant considerably compared with a conventional steam generation plant. The gas turbine may be connected to the compressor feeding pressurized air into the combustor.
The peripheral walls of the combustion chamber are cooled by recovering heat in a water/steam circulation. Additional heating surfaces, such as superheaters, reheaters and economizers, connected to the water/steam circulation are usually arranged in the combustion chamber. In circulating fluidized bed combustors the additional heating surfaces are arranged in the upper part of the combustion chamber. A multitude of steam piping, including risers and downcomers, thereby have to be arranged within the pressure vessel. Steam generation systems for power plants are therefore large even if pressurized.
The external pressure vessel can be a variety of shapes. Two common shapes are cylindrical and spherical. The price of a pressure vessel itself is high and the space inside the vessel must be utilized as advantageously as possible. The diameter of the pressure vessel should be kept as small as possible to minimize costs. The vessel wall thickness and hence material costs increase with the diameter of the vessel.
When pressurizing a circulating fluidized bed combustor system all of the combustion chamber, particle separator, fuel feeding and ash discharge arrangements, as well as the piping for the water/steam circulation are preferably arranged in one single pressure vessel. A conventional combustion chamber, having a square, rectangular or circular cross section, leads to a very space consuming arrangement, which needs a large diameter pressure vessel, leaving a large volume of unused space in the vessel.
The cost of the pressure vessel is a determining factor when calculating the total costs of the pressurized system. The bigger the system the more significant is the price of the pressure vessel.
It is therefore an object of the present invention to provide a pressurized circulating fluidized bed combustion or gasification system in which the size of the pressure vessel is minimized. This is achieved, according to the present invention, by utilizing in the pressurized combustion or gasification system a combustion chamber comprising a nonsymmetrical horizontal cross section, whereby at least two adjacent walls in the combustion chamber form an angle &gt; 90.degree., or the horizontal cross section of the combustion chamber is hemispherical.
The arrangement of combustion chamber equipment within the pressure vessel together with related auxiliary equipment including cyclones, filters, steam piping, fuel feeding or other equipment can be enhanced by utilizing unconventional combustion chamber shapes. According to the present invention a trapezoidal, semi-cylindrical, hybrid trapezoidal/semi-cylindrical, or other semicylindrical-approaching multisided (e.g. five or more sides) polygonal cross section is provided to better conform the shape of the combustor to the external vessel.
Advantages of the combustion chamber cross section of the invention include:
Optimal utilization of plan area within the external pressure vessel, thereby minimizing the size, cost, and space requirements of the vessel.
Minimization of the height of the combustor or gasifier, and of the external pressure vessel, by alternative configurations of the heat transfer surfaces. Such configurations include angling internal surfaces and maximizing wall area per unit height.
Maximization of the perimeter area of the combustor or gasifier, enhancing circulation characteristics of the combustor or gasifier if it is cooled.
Optimizing the cross sectional area of the combustor or gasifier, increasing the amount of usable space for location of heat transfer surfaces.
Reducing the potential effects of erosion by increasing the angle and/or rounding edges and corners within the combustor or gasifier to reduce eddies.
Increased wall area on the rear combustor wall for location of cyclone inlets, solids feeding or removal, and heat transfer surfaces.